1. Field of the Invention
The present invention relates a method for producing organosilicon polymer composed of a monofunctional component in the form of R.sub.3 SiO.sub.1/2 units and a tetrafunctional component in the form of the SiO.sub.4/2 unit. More specifically, the present invention relates a method for producing said polymer wherein the average molecular weight can be freely adjusted. This invention also relates to the composition produced by the method.
2. Prior Art Technologies and Their Drawbacks Requiring Solution
Organosilicon polymers composed of monofunctional siloxane units and tetrafunctional siloxane units have been known for some time and are currently employed in a broad range of applications. In a typical known method for producing said polymer, a water-soluble silicate salt such as water glass or sodium orthosilicate is converted into silicic acid monomer or silicic acid oligomer using hydrochloric or sulfuric acid and then trapped with trimethylchlorosilane, etc., after an appropriate polymerization as described by Daubt et al. in U.S. Pat. No. 2,676,182, issued Apr. 20, 1954 and by Currie et al. in U.S. Pat. No. 2,814,601, issued Nov. 26, 1957; alternatively, water is added to the mixture of an alkyl silicate and a hydrolyzable trialkylsilane and this cohydrolyzed as described by Goodwin in U.S. Pat. No. 2,857,356, issued Oct. 21, 1958.
This first method accrues the drawback of poor control of the rate of silicic acid polymerization which makes it impossible to produce organosilicon polymer with an arbitrary, desired molecular weight. In addition, the first method accrues the additional drawback that the produced silicic acid tends to polymerize with gelation since the alkali silicate salt must be neutralized with acid and the system then acidified because silicic acid is least stable at a pH of about 7. Furthermore, the first method suffers the drawback that the polymer product has a broad molecular weight distribution from low to high molecular weights.
On the other hand, the second method is a quite common method for synthesizing multicomponent organosilicon polymers in which hydrolyzable silanes with hydrolysis rates which are not significantly different are first mixed and then combined and cohydrolyzed with water. This method uses ethyl orthosilicate, ethyl polysilicate, methyl orthosilicate or methyl polysilicate as the alkyl silicate and typically uses trimethylchlorosilane as the hydrolyzable trialkylsilane. This method seeks to provide the unambiguous specification of the monofunctional siloxane unit/tetrafunctional siloxane unit ratio in the organosilicon polymer product by varying the molar ratio of the different hydrolyzable silanes which are first mixed with each other. However, in this method, alkoxy groups tend to remain at sterically complex sites in the organosilicon polymer product due to incomplete hydrolysis of alkoxy groups during the hydrolysis and condensation reactions of the alkyl silicate. This phenomenon even occurs with the use of a silicate with readily hydrolyzable alkoxy groups as the starting material, for example, methyl orthosilicate. A silicate with readily hydrolyzable alkoxy groups rapidly condenses with its partial hydrolyzate and a portion of the alkoxy groups remains unreacted and confined in the polymer and does not participate in the reaction. For this reason, this method necessarily accrues the drawback that the molecular structure and molecular weight of the polymer product differ significantly from those calculated from the charged monofunctional component/tetrafunctional component ratio.
Another method for making resinous copolymers is described by Lamoreaux et al. in U.S. Pat. No. 3,629,358, issued Dec. 21, 1971. The method taught by Lamoreaux et al. is complex involving hydrolysis of trimethylchlorosilane in the presence of ether with ammonium hydroxide; separating the layers; adding tetrachlorosilane, toluene, and methanol to the ether-silane layer; adding water to hydrolyze; and then recovering in the toluene layer a copolymer containing trimethylsiloxy units and SiO.sub.2 units. Such complex methods do not provide routes to predetermined molecular structures.
Mine et al. in U.S. Pat. No. 4,269,757, issued May 26, 1981, broadly teach resinous copolymers containing triorganosiloxy units and SiO.sub.2 units. The examples of Mine et al. show copolymers containing substantial amounts of alkoxy functional siloxane units, such as 12 mol % CH.sub.2 .dbd.CH(CH.sub.3 O).sub.2 SiO.sub.0.5 units Copolymers which contain large amounts of such alkoxy containing siloxane units would need to be made by some special technique or method. The patent of Mine et al. is an example of certain types of resinous copolymers, but only suggest their preparation by earlier methods, namely cohydrolysis.